Prefetch cleanup: (#2876)

- **Enable `enable_seqscan_prefetch` by default**
- Drop use of `seqscan_prefetch_buffers` in favor of
`[maintenance,effective]_io_concurrency`
This includes adding some fields to the HeapScan execution node, and
vacuum state.
- Cleanup some conditionals in vacuumlazy.c
- Clarify enable_seqscan_prefetch GUC description
- Fix issues in heap SeqScan prefetching where synchronize_seqscan
machinery wasn't handled properly.

.github/workflows/benchmarking.yml

@@ -232,7 +232,8 @@ jobs:
       if: matrix.platform == 'neon-captest-prefetch'
       run: |
         psql ${BENCHMARK_CONNSTR} -c "ALTER DATABASE neondb SET enable_seqscan_prefetch=on"
-        psql ${BENCHMARK_CONNSTR} -c "ALTER DATABASE neondb SET seqscan_prefetch_buffers=10"
+        psql ${BENCHMARK_CONNSTR} -c "ALTER DATABASE neondb SET effective_io_concurrency=32"
+        psql ${BENCHMARK_CONNSTR} -c "ALTER DATABASE neondb SET maintenance_io_concurrency=32"
       env:
         BENCHMARK_CONNSTR: ${{ steps.set-up-connstr.outputs.connstr }}
 

pgxn/neon/libpagestore.c

@@ -464,12 +464,12 @@ pg_init_libpagestore(void)
 							NULL, NULL, NULL);
 	DefineCustomIntVariable("neon.readahead_buffer_size",
 							"number of prefetches to buffer",
-							"This buffer is used to store prefetched data; so "
-							"it is important that this buffer is at least as "
-							"large as the configured value of all tablespaces' "
-							"effective_io_concurrency and maintenance_io_concurrency, "
-							"your sessions' values of these, and the value for "
-							"seqscan_prefetch_buffers.",
+							"This buffer is used to hold and manage prefetched "
+							"data; so it is important that this buffer is at "
+							"least as large as the configured value of all "
+							"tablespaces' effective_io_concurrency and "
+							"maintenance_io_concurrency, and your sessions' "
+							"values for these settings.",
 							&readahead_buffer_size,
 							128, 16, 1024,
 							PGC_USERSET,